This invention relates to apparatus for dispensing fluids, and in particular to such apparatus for dispensing small precisely metered amounts of fluid. The invention is particularly useful for the supply of metered amounts of fluid such as silver epoxy in semi-conductor manufacturing processes, but is not limited thereto.
In a number of manufacturing processes it is necessary to be able to dispense very small precisely metered amounts of sometimes relatively viscous fluid. One example of such a manufacturing process is in the semiconductor industry in which small precise amounts of silver epoxy must be dispensed onto a substrate prior to die attachment. The silver epoxy may be dispensed in the form of dots or continuous beads on the substrate, the precise amount and pattern varying depending on the size of the die and the product being formed.
A large variety of different types of dispensing apparatus have been proposed for dispensing small precisely metered amounts of fluid in industries such as the semiconductor manufacturing industry.
In a typical dispensing system, a time-pressure control unit is used to dispense fluid from a syringe. Fluid in the syringe is dispensed by compressed air that is controlled by the time-pressure unit. The syringe is mounted on a moving assembly and to dispense a dot or a continuous bead the assembly moves to the desired position horizontally and then down to the desired level. Pressure is applied to dispense the liquid epoxy.
Other types of dispensing apparatus may be termed positive displacement apparatus and may be termed diaphragm type and piston type.
An example of a known diaphragm type apparatus is described in detail in U.S. Pat. No. 4,974,754 (Wirz). This apparatus includes a metering chamber defined by a diaphragm that is operatively connected to an actuating device. An inlet opening and an outlet opening to the metering chamber are opened and closed by the diaphragm in response to movement of the actuating device. Such diaphragm devices have a number of disadvantages, however, including the fact that in the operating cycle time must be provided for refilling the metering chamber between dispensing operations, the fact that limited amounts of fluid can be dispensed per cycle, and the fact that worn parts need to be replaced in order to maintain dispensing accuracy.
Another known device may be considered to be an example of a piston type device. In this device two piston chambers are provided which define metering chambers for the fluid being dispensed. Fluid is dispensed from each of these piston chambers in turn with a valve that is moved between two positions so as to alternately connect the two piston chambers to an outlet. This valve is often a rotary or a sliding valve. This has an advantage over the diaphragm type device in that one piston is refilled while the other is being dispensed, but there is still an idle time while the valve is being moved between positions. Furthermore the valve can leak and bubbles may be created in the fluid when the valve is moved between piston chambers.
According to the present invention there is provided apparatus for dispensing metered amounts of a fluid, comprising: fluid supply means, an outlet and a fluid flow path between said supply and said outlet, wherein said fluid flow path comprises first and second metering chambers disposed in series and associated with first and second pistons for dispensing fluid from said chambers, wherein a first of said metering chambers disposed closest to said fluid supply means displaces a volume of fluid twice that of said second metering chamber disposed closest to said outlet.
In a preferred embodiment the metering chambers are generally tubular and are of an identical length, and wherein the fluid may be dispensed from the metering chambers by the movement of the associated piston rods therein, the piston rod associated with the first metering chamber having a diameter greater than the diameter of the piston rod associated with the second metering chamber by a factor of 2. The piston rods may have an identical length of travel between a home position and a limit position so that they may be operated by a single actuator.
Preferably the pistons move out of phase with each other into and out of said metering chambers such that fluid is dispensed from the two metering chambers alternately.
In a particularly preferred embodiment the fluid flow path comprises three switch valves operable to interrupt the fluid flow path, a first switch valve being provided between the fluid supply means and the first metering chamber, a second switch valve being disposed between the first and second metering chambers, and a third switch valve being disposed between the second metering chamber and the outlet.
In this arrangement, when the first and third switch valves are open and the second switch valve is closed, fluid is allowed to flow into the first metering chamber to fill the first metering chamber and at the same time fluid is dispensed from the second metering chamber to the outlet. When the first switch valve is closed and the second and third switch valves are open, half of the fluid in the first metering chamber is dispensed from the first metering chamber to the outlet and the other half of the fluid in the first metering chamber is used to fill the second metering chamber.
The apparatus may comprise a main body portion formed of two halves, with the fluid flow path being formed in one of the halves and the two halves being joined at an interface, and the three switch valves all being formed at the interface. A diaphragm may be located between the body portion halves, and means may be provided at the switch valves to urge the diaphragm into positions to open and close the fluid flow path.
According to another aspect of the present invention there is provided apparatus for dispensing metered amounts of a fluid, comprising: fluid supply means, an outlet and a fluid flow path between said supply and said outlet, wherein said fluid flow path comprises first and second metering chambers disposed in series and wherein fluid is alternately dispensed from said first and second metering chambers, and wherein when fluid is dispensed from the first metering chamber closest to said fluid supply means, a part of the fluid dispensed from the first metering chamber is used to fill the second metering chamber.